1. Field of the Invention
The present invention concerns an optical phase plate made or plastic sheet of film for use in optical control devices and, more particularly to an optical phase plate which is suitable to provide phase compensation for preventing coloration of screens in black and white liquid crystal display units.
2. Description of the Prior Art
In a transmission type black and white display, incident light is at first linearly polarized upon transmitting there of through a polarization plate, and then enters a liquid crystal cell to be changed into an elliptically polarized light. If the light passes through the polarization plate and is emitted in the form of the elliptically polarized light as it is, since the degree of elliptic polarization is different depending on the wavelength, the intensity of the transmitted light varies and this results in a coloration of the display. Accordingly, for preventing the coloration, phase compensation is necessary for reforming the elliptically polarized light again into linearly polarized light. In view of the above, a phase compensating liquid crystal cell is further laid over the liquid crystal display cell to obtain black and white display, but this involves problems increased weight and cost. The use of a molecularly oriented transparent plastic film as an optical phase plate instead of the phase compensating liquid crystal cell has also been considered.
The optical phase plate means according to this invention comprises a film or sheet-like material having birefringent nature, which causes a phase difference between optical rays orthogonal to each other upon therethrough due to the difference in refractive index in the two directions orthogonal to each other.
For phase compensation, in order to prevent the coloration of black and white liquid crystal display there is required a so-called .lambda.-plate, in which the phase difference between the wavelength of emitted light and the wavelength of incident light equals the wavelength of the incident light. Although application of the .lambda.plate to various optical applications has been studied, no satisfactory results have yet been obtained at present. As for the .lambda.plates proposed so far, there is known, for example, a uniaxially drawn optically anisotropic film or sheet having birefringence disposed by two or more, such that the main optical axes thereof are orthogonal to each other (Japanese Patent Laid-Open Sho 63-167304), or a single plate with no such stacking (Japanese Patent Laid-Open Sho 63-189804). Further, there is known an optical phase plate prepared by cutting birefringent crystals along a face slanted to the optical axis for compensating scattering of the wavelength of optical source (Japanese Patent Laid-Open Sho 62-218905), that is, an optical phase plate fabricated from birefringent material into a wedge-like shape.